Heretofore, for example, when welding together component parts of a vehicle or the like, a clamping device has been used for clamping the parts. The present applicant has proposed such a clamping device, having a main body portion, a cylinder connected to the main body portion, and a clamp arm, which is rotated a predetermined angle through a toggle link mechanism disposed in the interior of the main body portion under a driving action of the cylinder (see Japanese Laid-Open Patent Publication No. 2001-113468).
In such a clamping device, a piston and a piston rod of the cylinder are displaced in a axial direction under the action of a pressure fluid supplied to the cylinder, whereby a clamp arm is rotated through an operational angle based on a displacement amount of the piston via the toggle link mechanism that is connected to the piston rod, and the clamping device is switched between a clamped state, at which a workpiece can be clamped, and an unclamped state, at which the workpiece is released from the clamped state.
Further, in the clamping device, a detection member is connected with respect to a knuckle block, which is connected to the piston rod, whereby the rotational state of the clamp arm is detected by detecting the detection member by two proximity switches disposed at respective side portions of the body.
Recently, it has been desired to detect the rotational state of a clamp arm more easily and with greater accuracy.